1. Field of the Invention
The invention is directed to an improved fuel injection system for an internal combustion engine.
2. Description of the Prior Art
One fuel injection system of the type with which this invention is concerned is known from European Patent Disclosure EP 0 957 261 A1. This fuel injection system has one high-pressure fuel pump and one fuel injection valve, communicating with it, for each cylinder of the engine. The high-pressure fuel pump has a pump piston, which is driven to reciprocate by the engine and which defines a pump work chamber that can be made to communicate with a pressure chamber of the fuel injection valve, and this valve has an injection valve member by which at least one injection opening is controlled and which is movable by a pressure prevailing in the pressure chamber, counter to a closing force, in an opening direction for opening the at least one injection opening. A first electrically actuated control valve, by which a connection of the pump work chamber with a relief chamber is controlled, is provided. A second electrically actuated control valve is also provided, by which the pressure prevailing in a control pressure chamber, which pressure urges the injection valve member in the closing direction, is controlled. The control pressure chamber has a connection with the pump work chamber, and a connection of the control pressure chamber with a relief chamber is controlled by the second control valve. In this known fuel injection system, it is a disadvantage that the course of the fuel injection, i.e. the fuel quantity injected and the pressure at which the fuel injection takes place, can be varied only to a limited extent during one injection cycle. In particular in an injection cycle with a preinjection and an ensuing main injection, the pressure at which the main injection begins and the time interval between the main injection and the preinjection are coupled with one another and are not freely variable. If the main injection is to begin at a low pressure, then the time interval after the preinjection is short, and if the main injection is to begin at a high pressure, then the time interval after the preinjection is long.
The fuel injection system of the invention has the advantage over the prior art that by means of the blocking valve, even when the first control valve is open, an elevated pressure can be maintained in the pressure chamber and in the control pressure chamber, so that regardless of the switching state of the first control valve, a fuel injection can be controlled, particularly for a preinjection and/or a postinjection, by means of the second control valve. The pressure buildup for a main injection can be controlled by the first control valve, and the instant at which the main injection begins can be controlled by the second control valve. As a result, a decoupling between the pressure at which the main injection begins and the time interval since a preceding preinjection is made possible.
Other advantageous features and refinements of the fuel injection system of the invention are disclosed. One embodiment makes use of a simple the blocking valve possible. Another embodiment makes simple control of the pressure in the control pressure chamber possible. Another embodiment enables adjusting the fuel inflow into the control pressure chamber and the fuel outflow from the control pressure chamber, while another embodiment enables engine operation with low noise and low pollutant emissions. Another embodiment makes it simple to adjust the fuel quantity for the preinjection and to adjust the length of time that the first control valve is closed, while another embodiment makes it possible to adjust the fuel quantity for the preinjection simply and purely by mechanical means. Other variations make a postinjection possible without fuel having to be pumped by the pump piston during the postinjection, and embodiment make it simple to perform a preinjection, and enable relief of the pressure chamber and of the control pressure chamber.